Also, your mileage may vary. To one person it's all fine and to another they think it's the end of the world.
Being able to hold a line very well, and being confident in your bike handling skills are all crucial and helpful skills that go a long way to properly assessing danger and being ok with a given situation.
+1 to YMMV. I rode Cycle Greater Yellowstone's supported tour last year, and I was floored at comments. The ride description in the handbook as well as the web would tell anyone who read either, "These roads are open." But after the first day, it seemed like a full quarter of the riders had read neither. "OMG, there were TRUCKS!!! on the road!" "OMG, we must ride on the shoulders!" But I thought traffic was light, well-behaved, and rode the shoulders where they were good mostly so I didn't have to listen for traffic coming up from behind me; in other words, for my own convenience.
Sure, there are roads cris-crossing the counties near home. But you hit western states like Wyoming, and there is a road that goes from point A to point B. Maybe two, if you add in 30 miles or so. You can't close that road for bicyclists, so if you want to bicycle from A to B, you've got to ride with traffic on that road. Fortunately, traffic's usually pretty light. (For me, anyway.)